Precise metering of dry solid materials which flow similar to liquids, so-called fluidized or "floodable" solid materials, is an important but difficult task in many industrial applications and processes. Whenever such a fluidized solid material is continuously fed from a supply vessel of some sort into a production line, the rate at which material is metered to the process must be controlled, just as with any other material used as an ingredient or additive in a continuous or batch process.
However, these highly aerated or fluidized dry solid materials tend to flush through conventional metering mechanisms, preventing controlled feeding of the material into a process or production line. For example, many dry solid materials such as starch, cement, fly ash, stucco and cocoa, as well as a wide assortment of other fine, powdery products, have a tendency to become "fluid" and are difficult to control or meter after having been pneumatically transferred (where air is used as the conveying means) from one place to another. A typical example of this would be when such products are unloaded from a transport vehicle (rail car, ship, truck, etc) and into a storage hopper or silo. Until such time as the air entrapped within the material is dissipated, the material is generally very fluid and extremely difficult to handle or meter.
Another typical example would be when air is intentionally introduced into a dry solid material (when such material is stored in a hopper or silo) in order to promote the flow or discharge of the product when the product bridges or arches in the storage vessel and will not flow.
Another example of this problem includes the situation when gases are generated by the stored material itself due to a chemical reaction in progress. In fact, certain dry solid materials become very fluid at elevated temperatures because of gaseous emissions, resulting in similar materials-handling problems. Examples of such materials include hot stucco and lime.
As with other types of materials which must be metered into a continuous or batch process, fluidized solid materials are fed to such processes by one of two basic types of feeding systems - volumetric and gravimetric systems. The problems outlined above regarding the metering of fluidized solid materials are present regardless of whether a volumetric or a gravimetric feeding system is used.
As the name implies, volumetric systems dispense material by volume. They employ a displacement mechanism of some sort (for example an auger mounted below a supply vessel and feeding a fixed volume of material per revolution) operating at a set speed. Volumetric systems cannot always be relied on to produce good results, however, if actual conditions, such as the density, flow and handling characteristics of the materials change during the feeding process.
Where a high degree of accuracy is required, gravimetric systems are employed. These systems, also known as weight feeding systems, monitor their own feeding performance and automatically correct for variations from the desired or set feed rate. Gravimetric systems control the feeding of material by weight, thereby compensating for changes in density and/or irregular product flow characteristics. For this reason, these systems can provide much higher accuracy than volumetric feeders.
For weigh feeding dry solid materials including those that are highly aerated or fluidized, there are generally two types of gravimetric systems used: weigh-belt and loss-in-weight. Typical weigh-belt gravimetric feeding systems measure the weight of the material passing across the weigh-belt during operation, that is, while the belt is dispensing material. This measured weight is compared to an expected or "set" weight, causing the generation of a control signal. While such weigh-belt systems offer good accuracy for many uses, material sometimes accumulates on the belt or other critical components and thereby causes the control system to improperly adjust the feed rate. The problem is compounded if the accumulated material builds-up and then falls-off critical sections of the weighing mechanism causing shifts in the calibration of the scale's `zero` or calibration setting. Also, because of the flowing nature of the fluidized solid material, it is difficult to reliably keep the material on the belt at all times.
This type of gravimetric system, therefore, while using feedback principles to offer some control, has substantial inherent limitations that can seriously impair the accuracy of the device.
The other type of gravimetric feeding system usable for fluidized solid materials is the loss-in-weight system. A loss-in-weight system is a weigh feeding system in which the gradual decrease in weight of the system caused by the constant release of material from its scale-mounted supply vessel is monitored and compared to an expected decrease in weight to assess performance. Unlike the weigh-belt gravimetric systems, loss-in-weight systems are not affected by material adhering to the critical areas of the weighing mechanism, or a shift in the scale's `zero` or calibration, since the entire system is weighed continuously. In loss-in-weight systems, a control system continually (or with great frequency) monitors a signal indicating the decreasing total weight of the feed supply vessel and associated feed mechanism and adjusts the feed rate output to maintain an even rate of weight loss over time. If a weight loss amount over a given time period is larger than expected as sensed by the control system, it commands the feeding mechanism to slow down. Similarly, if the weight detected becomes too high, indicating that the weight loss has been less than expected, the control system orders an increase in the material output flow rate to cause the system to catch up with the expected feed.
Representative control systems for loss-in-weight feeding systems are shown and described in United States Re. Patents Nos. 32,101 and 32,102, and the patents cited therein.
Depending upon the accuracy of feed rate desired, volumetric, weigh-belt or loss-in-weight systems are used for feeding fluidized solid materials.
Prior attempts to prevent uncontrolled metering of fluidized solid materials include the use of a metering mechanism consisting of a special screw design or series of screws having an outer diameter sufficiently close to the inner diameter of the housing of the screw or series of screws such that the material cannot flush through the very small gap between the two. However, use of such screws has continued to result in problems because a highly aerated or fluidized solid material still flushes through the core of the screw in an uncontrolled fashion. Also, if a small solid item, such as a pebble, jams in the very small gap between the outer diameter of the screw and the inner diameter of the enclosing metering mechanism, the feeder ceases to operate. This jamming occurs because the special screws are not designed to handle large particle size materials or materials with solid impurities which can lodge in the gap. Further, such screws are difficult to manufacture and are expensive, resulting in an increased cost for the feeding system and replacement parts thereof.
Vibrating hoppers which feed material into a metering mechanism are also used to remove air from the material before the material typically enters the special screws described above. This solution also has problems, primarily for gravimetric systems, because the vibration of the hopper often causes widely fluctuating weight signals and results in inaccurate feeding. Further, a vibrating scale-mounted hopper is not always suited for longevity.
Therefore, there has been a need for a low-cost yet effective mechanism for metering fluidized dry solid materials while preventing the materials from flushing through the metering mechanism in an uncontrolled fashion.